Silicon wafers are used to manufacture photovoltaic cells. Each silicon wafer is cut from an ingot with a specialized saw. The surface of the wafers needs to be inspected for various defects. One type of defect is caused by the process of sawing-off the wafer from the silicon ingot. Saw grooves or saw marks are local, elongated, 3-dimensional departures from the normally flat surface of the wafer. Each saw mark is extended and formed in the sliding direction of the saw and parallel to said solar cell wafer in the surface of said solar cell wafer. In case of a cylindrical shaped ingot the saw marks run in the same direction as the saw is operated. In case of a cubical ingot the saw marks are almost parallel to one edge of the wafer. The length of the saw marks may vary between a few centimeters and the entire width of the wafer. Additionally, also the width and depth of the saw marks may vary. The form and status of a saw mark in a solar cell wafer influences the quality of the solar cell wafer.
Japanese patent application JP 2010-181328 A discloses a test equipment for a solar cell wafer surface which inspects a formation state of a saw mark in the surface of the solar cell wafer. The solar cell wafer being the subject of examination is conveyed by a conveyor in a direction parallel to the longitudinal direction of a saw mark formed in the surface. The test equipment is provided with a floodlight, a camera and a computer. The floodlight is arranged at an oblique direction to the surface and is adapted for lighting a sub-division to the surface of said solar cell wafer. The lighting direction perpendicularly intersects the longitudinal direction of the saw mark. The floodlight is a halogen lamp, a fluorescent lamp, etc., for example. A diffusion plate is used to diffuse the light for irradiating said solar cell wafer. The camera generates data of the picture of the surface of said solar cell wafer. The imaging direction (optical axis direction) of said camera may be an oblique direction to the surface of said solar cell wafer.
Japanese patent application JP 2008-134196 A discloses a technology wherein the picture of the surface of the solar cell wafer has dispersion in the light and darkness according to the grain boundary which exists in the solar cell wafer. In the picture of the surface of a solar, cell wafer, the brightness of the image of said saw mark is relatively low. Therefore, the image of said saw mark cannot be exactly identified as a candidate of a defect.
Japanese patent application JP 2005-345290 A discloses a technology, whereby a pixel of a predetermined number with high brightness is extracted from a division picture. In many cases, the problem is that an image of a saw mark cannot be exactly extracted from a picture of the surface of a solar cell wafer as a candidate for a defect. Therefore, it is difficult to set up a predetermined region centering on said pixel which distinguishes one image of said saw mark at a time.
The Japanese patent application JP 2000-046743 A applies inspection light to a wafer where recesses and projections in a specific direction are formed in parallel from a specific direction. A plurality of LEDs is arranged in a semi-circular shape, and a lighting direction is controlled by a lighting-controlling device. The lighting-controlling device sets a lighting direction in the initial state of a wafer on a turntable and successively adjusts the lighting direction for an angle based on a rotary angle signal from the encoder of the turntable. Cameras pick up the image of the edge part of the wafer every time the turntable stops at a specific angle. An image-processing device picks up an image from the cameras and detects a defect by image processing.
The German patent application DE 10 2009 010 837 A1 discloses a method for inspecting for the existence of sawing grooves on wafers utilized for manufacturing solar cells. The method involves projecting a light line on a wafer by a laser light source, and transporting the wafer on a transport device. The wafer is inspected during continuous transport of the wafer, where the wafer is arranged on the transport device such that sawing grooves are aligned at a right-angle to a transportation direction. A partial region of the wafer is examined such that images of the partial region are recorded using a surface camera.
The prior art methods show drawbacks. The manual methods are slow and do not inspect all solar cell wafers. As a result there is an insufficient detection of the defect (saw mark). Furthermore, many prior art methods are less accurate and less repeatable.
Some of the prior art methods only inspect part of the solar cell wafer, whereas saw grooves or saw marks can be present anywhere on the wafer. Particularly, also the deepest part of the saw groove can be anywhere on the wafer, too. As a consequence, the prior art methods may miss a groove completely or underestimate its depth. This also results in an insufficient detection of the defect.
There are some prior art methods which inspect the solar cell wafers in one direction only. However, the wafers are on a moving belt, and consequently, grooves perpendicular to the direction of transport of the belt can be detected and measured. In order to provide a sufficient detection these methods require operators knowing the direction in which the previous sawing process occurred. All wafers need to be placed in the same orientation onto the moving belt. As a consequence, the inspection process is slower, more error-prone, and less flexible.